Bar-strapped multifilar helix for traveling-wave tube



Aug. 22, 1961 D. A. WATKINS BAR-STRAPPED MULTIFILAR HELIX FOR TRAVELING-WAVE TUBE Filed July 2l, 1959 INI/ENTOR. DEAN A. WA Tlf/N5 arent Otticc 2,997,618 Patented Aug. 22, 1961 2,997,618 BNR-STRAPPED MULTIFIIAR HELIX FOR TRAVELING-WAVE TUBE Dean A. Watkins, Portola Valley, Calif., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed July 21, 1959, Ser. No. 828,675 4 Claims. (Cl. 31E-3.6)

This invention relates to a slow-wave structure for use in traveling-wave tube amplifiers, and more particularly to a bar-strapped multilar helix.

An object of this invention is to provide a slow-wave structure, which used with an appropriate high-voltage electron beam (for example l to 40 kilovolts) will form a traveling-wave tube amplier with the following characteristics: large 3-db bandwidth (about 2:1); and the tube will be relatively free from self-oscillation caused by beam-interaction with a backward space-harmonic of the structure having a velocity nearly synchronous with the electron beam, as compared with a single helix having a similar beam configuration and equal pitch and diameter. By the aforementioned description of bandwidth is meant to those skilled in the art that the frequences at the 3-db down points on the response curve are in the ratio of 2:1.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

FIG. l is a more or less schematic drawing of a traveling-wave tube showing one preferred embodiment of the invention;

FIG. 2 is an enlarged axonometric view of a barstrapped biiilar helix removed from the tube of FIG. 1 and partially broken away; and

FIG. 3 is a cross-section taken along the line 3-3 in FIG. l.

Reference is now made to the drawing. The particular multiiilar helix illustrated is a bilar helix made of two helices 2 and 4 of uniform conducting tape of width W and thickness T. Each has the form of a helix of constant pitch p (the axial distance in which the conductor makes one complete rotation about the axis), constant pitch angle \I/ (the complement of the angle between an element of the cylindrical surface in which the helix lines and the helical direction in that surface), and constant mean radius a. The helices are spaced equally from each other along their common axis.

The helices are connected together electrically (strapped) every distance d by means of identical conducting bars 6 (or wires, rods, etc), shown in FIG. 2 to be of circular cross-section with diameter D. The axes of the bars lie along diameters of the cross-section of the helix cylinder.

The helices and bars may be the same piece of metal formed into the shape described, or they may be separate pieces of metal formed as described and connected together to afford electrical connection and physical support between the separate pieces.

The spacing d between bars is shown in FIG. Z to be equal to half the pitch distance p. Other spacings are possible, in which case the bars would not be aligned with each other, in general, but would each be rotated about the axis of the structure from adjacent bars by some constant angle which depends upon d and \If.

The cross-sectional shapes of the conductors which form the helices and of the bars of secondary importance to the configurations of the conductors and their spacings relative to each other.

Thus, other cross-sectional shapes (e.g., circular,

oval, rectangular, etc.) of conductors would produce structures of quite similar characteristics.

The traveling-wave tube in which the helix of this invention is used is indicated generally at 8 and is ot conventional construction, including an electron gun 10, a collector 12, input terminals 14, and output terminals 16. The electron beam ycan travel in two segments, either inside the helix or outside of it, being directed in such a manner as to avoid hitting the bar straps or supporting structure. The helix can be supported by dielectric supporting rods inside of it (omitted for the sake of clarity) or can be supported by the connecting terminals alone if the tube is disposed with its axis vertical.

An understanding of the operation of this invention can perhaps best be had by review of the operation of an unstrapped bifilar helix. The unstrapped biilar helix has an infinity of possible modes of propagation, two of which are important here. The symmetric or pushpush mode is characterized by fields at the two helices being in phase with each other. It has a forward spaceharmonic (designated as a fundamental or O space-harmonic) which is useful for forward wave amplifiers, and a strong -2 backward space-harmonic which can cause oscillations when its velocity is nearly synchronous with the electron beam and therefore also nearly synchronous with the velocity of the fundamental space-harmonic.

An anti-symmetric or push-pull mode is also possible, for which the elds at the two helices are out of phase at any axial position, even at very low frequencies. This mode has a strong -1 backward spaceharmonic which can cause oscillations under the same near-synchronous condition with the beam and therefore also with the fundamental space-harmonic. The -1 and the 0 space-harmonics are synchronous at or near the frequency for which the circumference of the structure is equal to onehalf of a free-space wavelength. Recalling that a is the mean radius of the structure, the terminology common in Ithe art denes ka as the circumference measured in free-spaced wavelengths. Thus, the aforementioned synchronism of the '-l and O spaceharmonics, can be said to occur near ka=0.5. Synchronism between the -2 and the 0 space-harmonics occurs at or near ka= l.0.

The straps of the present invention connect the two helices together electrically, thereby forcing them to be in phase and prevent a push-pull mode with its -1 space-harmonic from propagating. The straps, because of their periodic perturbation of the unstrapped structure, also cause a stop band in the symmetric mode. By suitable choice of the strap spacing d there is produced a stop band which results in considerably higher beam currents being required for oscillation in the strapped structure than for interaction with the -2 space-harmonic of the unstrapped structure. The stop band is needed at about ka=l.0, which allows forward-wave interaction (traveling-wave tube operation) up through ka=0.5 and higher. 'Operation through such a high value of ka is one requirement for broadband amplification in a high power tube, the other requirement of the structure being low dispersion. Measurements have shown that the diametrically strapped bilar helix has very little more dispersion than a sheath helix of the same velocity up to ka=0.8.

Although the illustrated multitilar helix is a biilar helix, multilar helices using more than two such identical helices assembled along a common axis, equally spaced from one another, can be used. As the helices become more numerous, the width of each one would need to become smaller, so that the individual .helices do not touch each other. Diametric strapping of a multiflar helix having more than two separate helices would require more than the number of straps shown in the illustrated embodiment. They could meet at the axis of the structure and all 'lie in the same transverse plane, or they can be staggered along the axis. Bar strapping can also be accomplished without the bar passing through the axis of the structure. rIhus, the bars could connect only adjacent helices, so that the bars form a polygon in a transverse plane, or they could be staggered along the axis of the structure. Whatever the form of strapping, it must be periodic in the axial direction.

Obviously many modications and Variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A slow-wave structure for use in traveling-wave tubes comprising a multilar helix including a plurality of separate helices, each separate helix having the same constant pitch, the same constant pitch angle, and the same constant mean radius; said helices being spaced equally from each other along a common axis; and a plurality of conducting batrs, spaced periodically from each other along the direction of the axis of the multilar helix; each bar electrically connecting the helices together, lying across a diameter of the multilar helix, and intersecting the axis yof the multilar helix.

2. The slow-wave structure of claim l wherein the multilar helix is a bilar helix.

3.The slow-wave structure of claim 2 wherein the conducting bars are spaced from each other a distance equal to half the pitch distance of each separate helix.

4. In a traveling-wave tube, an electron gun, a ,co1- lector for electrons, and a slow-wave structure; said slowwave structure being the structure of claim 1.

References Cited in the tile of this patent UNITED STATES PATENTS 2,768,322 Fletcher Oct. 23, 1956 2,774,005 Kazan Dec. 11, 1956 2,802,135 Dodds Aug. 6, 1957 2,806,975 Johnson Sept. 17, 1957 2,809,321 Johnson et al. Oct. 8, 1957 2,846,613 Pierce Aug. 5, 1958 2,859,375 Brewer Nov. 4, 1958 Y2,889,487 Birdsall et al. June 2, 1959 Patent No@ 2,997,618 August 22, 1961 (SEAL) UNITED Y STATES PATENT. OFFICE l CERTIFICATE OF CORRECTION Dean. A., Watkins It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 14, after "which" insert when line L18 for "lines" read 11es same column 1, line 69, after "bars" insert are column 2, line 39, for "free-spaced" read free-space Signed and sealed this 6th day of March 1962e Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

